Germany's main opposition leader vowed on Thursday to impose immediate border controls if he becomes chancellor, as expected, after elections next month, a day after an Afghan asylum seeker was arrested for a deadly knife attack targeting children.

Germany's opposition leader has vowed to bar people from entering the country without proper papers and to step up deportations if he is elected as chancellor next month, as a knife attack by a rejected asylum-seeker spills over into an election campaign in which he is the front-runner.